Induction motor braking system

ABSTRACT

A braking system for a polyphase induction motor fed from a source of alternating current through a controlled rectifier converter having a variable output frequency. A control system is provided which is capable of controlling the output frequency of the converter relative to the rotor speed of the motor such that the motor can be operated with either a positive or a negative slip frequency. When it is desired to brake the motor it is operated at a negative slip frequency and braking resistors are connected across the phase windings of the motor. In addition the negative slip frequency is varied during the braking mode of operation as a function of induction motor rotor speed. The system is capable of reducing the conduction angle of the controlled rectifier converter during braking and is capable of connecting in different valued braking resistors for different speed ranges of the motor. The braking system is disclosed herein for use with a vehicle propelled by an induction motor and more particularly an induction motor powered off-highway earthmover.

United States Patent Salihi et al.

3,688,171 1 1 Aug. 29, 1972 Inventors: Jalal T. Salihi, Birmingham; JohnJ.

Brockman, Southfield; George J. Spix, Clawson, all of Mich.

General Motors I Corporation, Detroit, Mich.

Filed: Aug. 13, 1971 Appl. No.: 171,464

Assignee:

[52] US. Cl. ..318/211, 318/227, 318/230, 318/231, 318/376 [51] Int. Cl...I-I02p 3/22 [58] Field of Search ..318/209, 211, 227, 230, 231,318/375, 376

[56] References Cited UNITED STATES PATENTS 3,153,182 10/1964 Choudhury..318/211 3,293,520 12/1966 Lehry ..318/227 3,590,351 6/1971 Littwin..318/211 Primary Examiner-Gene Z. Rubinson 'Att0mey.E. W. Christen etal.

[57] ABSTRACT A braking system for a polyphase induction motor fed froma source of alternating current through a controlled rectifier converterhaving a variable output frequency. A control system is provided whichis capable of controlling the output frequency of the converter relativeto the rotor speed of the motor such that the motor can be operated witheither a positive or a negative slip frequency. When it is desired tobrake the motor it is operated at a negative slip frequency and brakingresistors are connected across the phase windings of the motor. Inaddition the negative slip frequency is varied during the braking modeof operation as a function of induction motor rotor speed. The system iscapable of reducing the conduction angle of the controlled rectifierconverter during braking and is capable of connecting in differentvalued braking resistors for different speed ranges of the motor. Thebraking system is disclosed herein for use with a vehicle propelled byan induction motor and more particularly an induction motor poweredoff-highway earthmover.

10 Claims, 7, Drawing Figures l'HlMl Ml )VLR CONVERTER GATE FIRINGCONTROL 1 322 ACCLERATOR I CONTROL P114 VOLTAGE TO TFcReiig/rllccgFREQUENCY 112 l CONVERTER CONVERTER l 76 MOTORING g PROGRAMMER 440 g x81 6 24 NOV H BRAK'NG CONVERTER 'NVERTER PROGRAMMER PATENTEnAuszs m2SHEET 2 BF 5 0mm 00m OfiN Om ON Om 1N VENTORS m Y, Mm .51% WNW W.

INDUCTION MOTOR BRAKING SYSTEM This invention relates to electricalbraking of an induction motor and more particularly to a system forbraking an induction motor powered vehicle such as an earthmover.

A well known characteristic of an induction motor is that if the machineis operated such that the rotor speed exceeds the synchronous speed ofthe motor as dictated by the frequency of the applied voltage theinduction machine will operate as an induction generator andconsequently will apply a braking force to the rotor of the motor.During this braking mode of operation the induction machine is operatingwith a negative slip frequency since the frequency related to rotorspeed is higher than the frequency of the applied voltage.

Induction motor-control systems having been devised where the slipfrequency of the system is controlled and where the slip frequency ismade negative to cause the motor to operate as a generator and thereforein a braking mode. In these systems energy has been returned to thevoltage source through the frequency control device connecting thevoltage source and the motor. An example of a system that uses negativeslip frequency for braking is disclosed in the patent to Lehry U.S. Pat.No. 3,293,520.

Induction motor braking systems have also been devised where braking isaccomplished by disconnecting the power source from the motor,connecting capacitors with the phase windings of the motor to cause themotor to regenerate and subsequently connecting resistors with the phasewindings. Such a braking system is disclosed in the patent to ChoudhuryUS. Pat. No. 3,153,182.

In contrast to the just mentioned braking systems it is an object ofthis invention to provide an induction motor braking system wherein theinduction machine is operated at a negative slip frequency to initiatebraking but wherein power is not returned to the power source duringbraking. Instead of returning power to the voltage source resistors areconnected across the phase windings of the motor when it is operated ata negative slip frequency. In addition the negative slip frequency isvaried as a function of rotor speed to maintain 'a predeterminedrelationship between the impedance of the induction machine and thebraking resistor to thereby provide optimum braking. With thisarrangement the triggering of the converter connecting the power sourceand the motor is simplified since power need be transferred in only onedirection. The system nevertheless provides either a braking or powermode of operation for the motor with control of slip frequency eitherpositive or negative.

Another object of this invention is to provide a braking system of thetype described wherein the induction motor is supplied from a source ofalternating current through a controlled rectifier converter and whereinthe conduction angle of the controlled rectifiers of the converter arereduced during the braking mode of operation. As an example, theconduction angle may be reduced from l20for power operation to 60 forbrak ing operation.

power to the source. In carrying this object forward the motor isoperated with a negative slip frequency and braking resistors areconnected across the phase windings of the motor. The negative slipfrequency is varied as a function of motor speed and no power isreturned to the source of alternating current during the brakingoperation.

Still another object of this invention is to provide a braking system ofthe type described wherein the voltage applied to the motor iscontrolled during the negative slip braking mode. Where the system ispowered by an alternating current generator the field current of thegenerator is controlled during braking to control the voltage applied tothe induction machine which is then operating as a generator.

A further object of this invention is to provide a braking system for aninduction motor powered vehicle such as an earthmover. In carrying thisobject forward the vehicle is provided with a manually operable brakingcontrol which when operated by the vehicle operator shifts the inductionmotor from a power mode to a braking mode. Actuation of the brakingcontrol by the operator causes the motor to operate with a negativeslip, switches in the braking resistors and causes the negative slip tovary as a function of rotor speed.

Still another object of this invention is to provide an induction motorbraking system of the type described wherein the resistance value of thebraking resistors is varied for different speed ranges of the motor. Inaddition the system includes means for providing different negative slipfrequency motor speed functions for the different speed ranges matchedto the resistance value of the braking resistance.

In the drawings:

FIG. 1 is a system diagram of an induction motor power and brakingsystem made in accordance with this invention;

FIG. 2 is a schematic circuit diagram of the converter shown in blockdiagram form in FIG. I;

FIG. 3 is a voltage waveform timing diagram illustrating the periods oftime in which the phase windings of the induction motor of thisinvention are energized;

FIG. 4 is a logic circuit diagram illustrating the logic for controllingthe firing of the controlled rectifier converter shown in FIG. 2;

FIG. 5 illustrates voltage waveforms showing the various voltages andtheir timed relationship developed by the logic system shown in FIG. 4;

FIG. 6 is a curve of motor speed versus negative slip frequency providedby the system of this invention during braking of the induction motor;and

FIG. 7 is a schematic circuit diagram of a braking system made inaccordance with this invention where different value resistors are usedin the braking system and wherein the system switches in various brakingprogramming circuits for each value of resistance used in braking theinduction motor.

The induction motor braking system of this invention will be describedin conjunction with an electrically propelled vehicle and moreparticularly an electrically propelled earthmover. It is to beunderstood; however, that the braking system of this invention could beused in other environments and may for example be used in a plant orfactory where a commercial source of alternating current is available.

Referring now to the drawings and more particularly to FIG. 1, abrakingsystem for an induction motor made in accordance with thisinvention is illustrated for use with an electrically propelled vehicle.In FIG. 1 the reference numeral indicates a prime mover of a motorvehicle which may be for example a diesel engine or a turbine. Theoutput shaft of the prime mover 10 is coupled to the rotor of analternating current generator which is generally designated by referencenumeral 12. The rotor in FIG. 1 is illustrated as a field winding 14which is suitably wound on the rotor of the alternating currentgenerator 12 and which as will become more apparent hereinafter controlsthe output voltage of the alternator 12 when the field current in thefield winding 14 is varied. The alternator may be of the brushless typeif desired in which case the field 14 would be fixed. The alternatingcurrent generator 12 has a three-phase Y-connected output windingdesignated by reference numeral 16 and this winding may be wound on asuitable stator core as is well known to those skilled in the art.

The three-phase Y-connected output winding 16 is connected with powersupply conductors designated by reference numerals 18, 20 and 22. Thesepower supply conductors 18, 20 and 22 are connected with a converterwhich is generally designated by reference numeral 24 and which isillustrated in detail in FIG. 2. The

converter 24 is comprised of 36 controlled rectifiers which operate toconvert the alternating current appearing at conductors 18, 20 and 22 tosquare wave alternating current (FIG. 3) which is applied to phasewindings of an induction motor. To this end the output conductors of theconverter are connected respectively with conductors 26, 28, 30, 32, 34and 36. The output conductors that have just been described feed thephase windings of a three-phase Y-connected induction motor which isgenerally designated by reference numeral 38. The phase windings of theinduction motor 38 are designated by the letters A, B and C. It can beseen that conductors 26 and 28 feed the phase winding B, conductors 30and 32 feed the phase winding A and conductors 34 and 36 feed the phasewinding C. The induction motor 38 has a squirrel cage rotor which isdesignated by reference numeral 40.

When it is desired to operate the induction motor as a brake, a brakingresistor designated by reference numeral 42 is connected in parallelwith the phase winding A through a relay controlled switch designated byreference numeral 44A. The relay controlled switch 44A is controlled bya relay coil designated by reference numeral 44 which will be describedin more detail hereinafter. The resistor 42 is paralleled by a capacitordesignated by reference numeral 46. In a similar fashion the phasewinding B is connected in parallel with a braking resistor 48 which isconnected across a capacitor 50 and in series with relay controlledcontact 44B. The phase winding C in a similar manner is paralleled bybraking resistor 52, capacitor 54 and relay controlled switch designatedby reference numeral 44C. As will become more readily apparenthereinafter, the relay controlled switches 44A, 44B and 44C are allclosed when it is desired to operate the induction motor 38 in a brakingmode.

The squirrel cage rotor of the induction motor is coupled to the drivewheels 56 and 58 of the vehicle by a shaft 60 and a differentialdesignated by reference numeral 62. The drive wheels 56 and 58 may befor example a pair of wheels on an articulated earthmover. The otherpair of wheels on such an earthmover (not illustrated) may be driventhrough another induction motor connected with another converter fedfrom the alternating current generator.

The motor control system of this invention is capable of controlling theslip frequency of the induction motor 38 and is capable of providing apositive slip frequency for motoring operation and a negative slipfrequency when it is desired to brake the induction motor and slow downthe earthmover. To this end the motor control system is provided with amagnetic pickup device generally designated by reference numeral 64 forsensing the speed of rotation of the rotor 40 of the in duction motor.This magnetic pickup device 64 comprises a toothed wheel designated byreference numeral 66 and a pickup coil which is designated by referencenumeral 68. The voltage generator or magnetic pickup 64 is arranged suchthat a series of voltage pulses are induced in the pickup coil 68, thefrequency of which is a function of the speed of rotation of the rotor40. The voltage pulses developed in coil 68 are applied to a conductor70 and the frequency of these voltage pulses is designated by the letterF,,,. The frequency of these voltage pulses are used in determining theslip frequency of the induction motor 38 as will be explained. Thevoltage pulses on line 70 are applied to a pair of frequency-to-voltageconverters which are designated by reference numerals 71 and 72. Thesefrequency-tovoltage converters may take any well known form which iscapable of developing a direct output voltage, the amplitude of which isa function of the frequency of the voltage pulses applied thereto fromline 70. The direct voltage output of the frequency-to-voltage converter72 is applied to a braking control device or braking slip programmerdesignated by reference numeral 74 and is also applied to a motor slipprogrammer designated by reference numeral 76. It therefore will beapparent that a direct voltage is applied to the braking control device74 and to the motoring programmer 76 the magnitude of which is afunction of the speed of rotation of the rotor 40 of the induction motorand this voltage is designated as F The voltage F,,,, the magnitude ofwhich is a function of induction motor rotor speed, is combined witheither a positive voltage or a negative voltage to provide either acontrolled positive slip frequency or a negative slip frequency for thebraking mode of operation of the induction motor 38. In order toaccomplish this the conductor 77 which has a direct voltage F m (rotorspeed) applied thereto is applied to a direct voltage adder-subtractor79.

The adder-subtractor 79 has another input from line 80 which isconnected with a movable contact 44D of a relay operated switchcontrolled by relay coil 44. This switch has a fixed contact 44Econnected with the output of motoring programmer 76 and a fixed contact44F connected with the output of an inverter 81 connected betweencontact 44F and braking programmer 74.

With contact 44D engaging contact 44E as shown in FIG. 1 the system isset to provide motoring operation. The system is arranged such thatunder this condition of operation a positive voltage F, from motoringprogrammer 76 will be added to a positive voltage F on conductor 77 withthe result that a direct voltage F F, appears at the output ofadder-subtractor 79 on conductor 82. This voltage F,, F, on conductor 82will set the switching frequency of converter 24 to set the positiveslip frequency of motor 38 in a manner to be more fully describedhereinafter.

When movable contact 44D engages fixed contact 44F the system is set forthe braking mode of operation. The output voltage of braking programmer74 is inverted by inverter 81 and a negative voltage F is new combined(algebraic sum) with positive voltage F,,, in adder-subtractor 79 withthe result that a voltage F, F, is applied to line 82. This causes themotor to operate with a negative slip frequency to cause the motor tooperate in a braking mode.

The relay coil 44 is connected in series with a source of direct current102 and a switch 104 which is mechanically coupled to a manuallyoperable brake controller which is illustrated as a brake pedaldesignated by reference numeral 106. The switch 104 is normally open butwhen the brake pedal 106 is depressed a predetermined amount, the switch104 is closed to energize the relay coil 44. When relay coil 44 isenergized, the switch contacts 44A, 44B and 44C are closed to connectthe braking resistors across the phase windings of the motor. Inaddition, when relay coil 44 is energized the contact 44D is moved fromits position contacting contact 44E to a position engaging fixed contact44F. This means that when the brake pedal is depressed the negativevoltage F, at contact 44F will be combined with the positive voltage Fby adder-subtractor 79. On the other hand, when the brake pedal 106 isfully released and switch 104 is open the contact 44D engages contact44E to provide motoring operation. The relay coil 44 further operatescontacts in a gate firing control circuit designated by referencenumeral 107. These contacts are illustrated in FIG. 4 where they aredesignated by reference numerals 446 and 44H. The dotted line 110 shownin FIG. 1 and in FIG. 4 indicates that these contacts are operated bythe energization of relay coil 44. As will be more fully describedhereinafter, the relay switches 44G and 44H are closed whenever thebrake pedal 106 is depressed in order to reduce the conduction angle ofthe controlled rectifiers of converter 24.

The voltage on conductor 82 will be the sum of F, F, or will be equal tothe difference F F, depending upon whether or not the brake pedal 106 isdepressed. The voltage on conductor 82 is a direct voltage and isapplied to a conventional voltage-to-frequency converter designated byreference numeral 112. The output of the voltage-to-frequency converter112 is a series of pulses applied to line 114 which have a frequencythat is a function of the direct voltage appearing on conductor 82. Whenmotoring operation is being utilized the frequency of the pulses onconductor 114 will be a function of F,, F On the other hand, when thebraking programmer 74 is supplying the conductor 82 the frequency of thevoltage pulses on conductor 114 will be a function of F F,,. The pulseson conductor 114 are applied to a gate firing control circuit designatedby reference numeral 107 and this control circuit is illustrated indetail in FIG. 4.

The output of the gate firing control circuit 107 is applied to aconductor 115 which is connected to the controlled rectifier converter24. As will be more fully described hereinafter, the output of the gatefiring control circuit 107 is a series of pulses which sequentially gategroups of controlled rectifiers conductive at a frequency determined bythe pulse rate applied to conductor 114. This frequency, as is morefully described hereinafter, is determined by whether or not theinduction motor is operating in a motoring or power mode or in a brakingmode where a negative slip frequency condition exists.

The brake pedal 106 is connected with a variable resistor designated byreference numeral 116. This variable resistor 116 is connected with afield current control device designated by reference numeral 118 whichmay take the form of a conventional voltage regulator which adjusts thefield current supplied to the field winding 14 as a function of theamount of depression of the brake pedal 106.

The voltage regulator 118 senses the output voltage of alternator 12 bymeans of transformer 119 and bridge rectifier 121 which applies a directvoltage to regulator 118 that is a function of the magnitude of theoutput voltage of generator 12. This voltage is compared with a voltageset by the position of resistor 116 with the result that the outputvoltage of the alternator is regulated to a value determined by thesetting of variable resistor 116. Thus, as the brake pedal 106 isdepressed the field current applied to the field winding 14 is regulatedto increase the output voltage of the alternator l2 and thereforeincrease the output power of the alternator 12. The field currentcontrol device 118 will maintain the output voltage of the alternator 12at a predetermined value which is determined by the amount of depressionof the brake pedal 106. As will be more fully described hereinafter,this preset voltage determines the amount of braking that takes place inthe induction motor 38 when the induction motor is operated in itsbraking mode.

Referring now more particularly to FIG. 2, a schematic circuit diagramis illustrated of the converter which is designated by reference numeral24 in FIG. 1. In FIG. 2 certain parts of the system that have beenillustrated in FIG. 1 are repeated and these parts have been identifiedwith the same reference numerals as were used in FIG. 1. The converter24, as previously described, comprises 36 controlled rectifiers whichare illustrated in FIG. 2. These controlled rectifiers are connected insix three-phase full-wave bridge rectifier networks to supplysubstantially square wave alternating voltages to the phase windings A,B and C of the induction motor 38. These controlled rectifiers aredesignated in groups of three by reference numerals A, 122A, 124A, 126A,1283, 130B, 132B, 134B, 136C, 138C, 140C and 142C. Consideringcontrolled rectifiers 120A and 122A it will be appreciated that thesecontrolled rectifiers are connected in a threephase full-wave bridgerectifier network having AC input terminals designated by referencenumeral 144 which are connected respectively with the power supplyconductors 18, 20 and 22. The output terminals of this three-phasefull-wave bridge rectifier network have been designated by referencenumerals 146 and 148 and it is seen that these direct current outputterminals are connected with conductors 30 and 32 which feed the phasewinding A of the induction motor 38. It therefore will be appreciatedthat when gate signals are simultaneously applied to the gate electrodesof controlled rectifiers 120A and 122A, a full-wave bridge rectifiercircuit is provided which provides a positive voltage on conductor 32and a negative voltage on conductor 30. This voltage is designated bythe letter A+ in FIG. 3 which indicates the time in which the controlledrectifiers that supply the A+ voltage, namely, controlled rectifiers120A and 122A are biased conductive to apply a positive voltage to thephase winding A. It is noted that this voltage extends for 120electrical degrees and this gate firing arrangement is provided by thelogic system illustrated in FIG. 4 to be described hereinafter.

The negative voltage for phase winding A, and designated as A- in FIGS.2 and 3, is provided whenever the control rectifiers 124A and 126A aresimultaneously gated conductive. This will provide a direct voltage tophase winding A which is positive on conductor 30 and negative onconductor 32 and this is shown as a negative voltage A in FIG. 3 whichoccurs between 180 and 300 electrical degrees.

The groups of controlled rectifiers are gated conductive in accordancewith the voltage timing curves illustrated in FIG. 3. Thus, it can beseen that from zero to 60 electrical degrees the control rectifiers 120Aand 122A will be gated conductive to provide the first 60 of the A+voltage shown in FIG. 3. At the same time, the B- voltage shown in FIG.3 will be provided by gating controlled rectifiers 1323 and 134Bsimultaneously conductive to energize the phase winding B of theinduction motor 38. The remainder of the conduction periods for thegroups of controlled rectifiers shown in that the gating arrangementshown in FIG. 3 for the controlled rectifiers shown in FIG. 2 is anonsynchronized system in that the controlled rectifiers are gatedconductive without regard to the instantaneous phase relationship of thevoltages developed by the output winding 16 of the alternator. It willbe further appreciated by those skilled in the art that by thisnonsynchronized triggering arrangement it is not possible for theinduction motor 38, when operating at a negative slip frequency as aninduction generator, to supply power to, the power lines 18, 20 and 22through converter24. It also should be pointed out that the converter 24is a frequency changer in that it will supply a square wave alternatingoutput voltage to the phase windings of the motor 38 the frequency ofwhich is a function of the repetition rate of the pulses applied toconductor 114 shown in FIG. 1. This means that the output frequency ofconverter 24 will be different from its input frequency. This repetitionrate can provide either a positive controlled slip frequency for theinduction motor or a negative slip frequency for braking as is morefully described hereinafter.

Referring now more particularly to FIG. 4, the gate firing control orlogic system designated by reference numeral 107 will now be described,it being understood that the circuitry shown in FIG. 4 represents theblock 107 shown in FIG. 1. In FIG. 4 the reference numeral 114 againdesignates a conductor which has a series of pulses applied theretowhich is determined by the output of the voltage-to-frequency converter112 shown in FIG. 1. The three-phase logic system shown in FIG. 4includes a flip-flop which is designated by reference numeral 250. Theflip-flop 2511 has terminals designated by the letters S, T, R1, R2, 0and O, which represent set, trigger, reset terminals and outputterminals for the flip-flop. The flip-flop 250 together with flip-flops252 and 254'form a conventionalring counter when they are electricallyconnected, as shown in FIG. 4. It is seen that the flip-flops 252 an d254 have terminals designated by letters S, T, R, Q and Q. Theseflip-flops are known to those skilled in the art and further descriptionof these flip-flops is therefore considered unnecessary. It is seen thatthe pulses on conductor 114 are applied to the trigger terminal T of theflip-flop shown in FIG. 4, voltages are developed at junctions 256, 260and 264 which are depicted in the timing chart of FIG. 5. Voltages whichare developed at junctions 258, 262 and-266 are of the same duration orpulse width as those developed respectively at junctions 256, 260 and264 but of an opposite polarity.

From an inspection of FIG. 4 it can be seen that the junction 266 isconnected by conductor 270 with one of the inputs of a three-input NANDgate designated by reference numeral 268. The terminal 256 of the ringcounter is connected with another input of the NAND gate 268 by aconductor 272. The output of the NAND gate 268 is applied to the inputof a single input NAND gate (inverter) designated by reference numeral274. The output of NAND gate 274 is applied to the base of an NPNtransistor designated by reference numeral 276. The emitter oftransistor 276 is coupled to the base of another NPN transistor which isdesignated as QA+. The conduction period of transistor QA+ isillustrated in the timing diagram of FIG. 5 and during motoringoperation of the induction motor this conduction period will be Theoutput voltage developed when transistor QA+ is conductive is utilizedto simultaneously gate controlled rectifiers 120A and 122A conductive.This voltage can be coupled to the gate electrodes of the just mentionedcontrolled rectifiers by a transformer in a manner which is fullydisclosed in patent application Ser. No. 57,143, filed on July 22, 1970in the name of Jalal T. Salihi et al. and assigned to the assignee ofthis invention and now US. Pat. No. 3,611,104. It is pointed out thatthe conduction period of transistor QA+ will be 120 during normal poweror motor operation of the induction motor 38 but this conduction angleis reduced to 60 when the relay operated switches 44G and 44H are closedas will be more fully described hereinafter. The 60 conduction periodfor transistor QA+ is also illustrated in the FIG. timing chart.

It is seen in FIG. 4 that there are five other three input NAND gateswhich are identical with the NAND gate 268 and each of these NAND gateshas been designated by the same reference numeral. The remaining NANDgates have pairs of input terminals which have been designated by thesame reference numerals as used for the outputs of the flip-flops 250,252 and 254. It is to be understood that the input terminals of the NANDgates 268 of these circuits are connected with the output terminalsdesignated by like reference numerals of the flip-flops of the shiftregister by conductors which are not illustrated. The output transistorsof each of these circuits or the remaining five circuits have beendesignated by letters QB+, QC+, QA-, QB- and QC-, it being understoodthat these transistors respectively control the switching of likedesignated groups of controlled rectifiers of converter 24. It is-seenin FIG. 4 that the trigger circuit for the five remaining transistors isnot completely illustrated in that it eliminates the NAND gate 274 andthe intermediate transistor but it is to be understood that the outputof each NAND gate 268 is coupled to a respective transistor by the samecircuitthat couples the NAND gate 268 and the transistor QA+. It is tobe further understood that the remaining five transistors are coupled tothe groups of controlled rectifiers by circuitry of the type shown inthe above mentioned copending patent application Ser. No. 57,143.

As long as the relay control switches 44G and 44H are in their openposition, as shown in FIG. 4, the groups of controlled rectifiers of theconverter 24 will be gated conductive for 120 conduction periods formotor operation, as'is illustrated in FIG. 5. When the brake pedal 106is depressed, however, the relay coil 44 is energized and the contacts44G and 44H controlled thereby and as shown in FIG. 4 are moved from anopen position to a closed position. This reduces the conduction angle ofthe various controlled rectifiers of converter 24 to 60 as is depictedin FIG. 5 of the timing diagram identified as braking operation.

In order to accomplish this result, the system of FIG. 4 includes apulse control circuit which is generally designated by reference numeral280. The control circuit 280 is comprised of three identical NAND gates,each of which is designated by reference numeral 282. The NAND gates 282are connected in a manner shown in FIG. 4 with a capacitor 284 connectedbetween conductor 286 and ground.

The input to control circuit 280 is formed by a conductor 288 which isconnected with the R2 terminal of flip-flop 250. The output of thecontrol circuit 280 is by way of conductor 290 which is connected withthe D, tenninal of a flip-flop designated by reference numeral 292. Theflip-flop 292 is a 60 flip-flop and has terminals designated as D S, T,R, Q and Q. The D, terminal of the flip-flop 292 may be termed a directset terminal and the flip-flop 292 can be of any well known commerciallyavailable type and may be, for example, a Fairchild U6A99485l flip-flop.It is seen that the trigger terminal T of the flip-flop 292 is coupledto the conductor 114 via a conductor 294. It therefore will be apparentthat the trigger pulses from the voltage-tofrequency converter 112 shownin FIG. 1 are applied to the trigger ter r ninal T of the flip-flop 292.The set terminal S and Q terminal of the flip-flop 292 are connectedwith a conductor 296 which in turn is connected to one side of the relayoperated switch 44H. It is further seen that the reset terminal R andthe Q terminal of the flip-flop 292 are connected with a conductor 298which in turn is connected to one side of the relay operated switch Thevoltages across the output terminals Q and Q of flip-flop 292 are shownin FIG. 5. The opposite side of the relay operated switch 446 isconnected to a conductor 300 which goes to respective input terminals ofthree NAND gates 268 the other three NAND gates being fed by conductor301.

When the relay operated switches 446 and 44H are closed as when thebrake pedal 106 is operated to close switch 104 shown in FIG. 1 thetransistors QA+, QB+, QC+, QA, QB-, and QC- will be gated conductive forperiods of time illustrated in FIG. 5 denoted by the legend brakingoperation. It can be seen as compared to motor operation that theconduction angle of the groups of controlled rectifiers has been reducedfrom a to 60. This has been found to provide at least two advantages inoperation of the braking system. First of all, by reducing theconduction angle to 60 the power supplied by the generator duringbraking is reduced. In addition, it has been discovered that thelikelihood of false triggering of the groups of controlled rectifiersthat make up the converter 24 is reduced when the conduction angle isreduced to substantially 60 during braking of the induction motor.

The voltage V280 shown in FIG. 5 is the output voltage from the controlcircuit 280 shown in FIG. 4 which is applied to conductor 290. Thisvoltage provides synchronization for the system and'synchronizationoccurs when the voltage drops to zero, as shown in the waveform of FIG.5.

Referring now more particularly to FIGS. 1 and 6, a description will nowbe made of the braking programmer 74 for programming the negative slipfrequency of the induction motor during a braking mode of operation. Thebraking programmer 74 can take the form of an operational amplifierwhich receives a direct voltage of a magnitude F at its input terminalsand develops an output voltage F which varies linearly with the speed ofrotation of the rotor 40 of induction motor. This output voltage hasbeen designated as F,, in FIG. 1 and this voltage can be stated as equalto a constant K multiplied by F or in other words equal to K F Thisvoltage F, is illustrated in FIG. 6 where the relationship between motorspeed F,, and voltage F are shown in graphical form. It is seen fromFIG. 6 that when the induction motor is operating at approximately 6,000rpm the negative slip frequency will be approximately 1 cycle persecond. The braking programmer 74 which, as stated above, could be anoperational amplifier provides the proper voltage relationship betweenthe motor speed signal F,, and the negative slip frequency signal Fwhich is equal to K F,,.. It is seen from FIG. 6 that as the motor speedincreases the negative slip frequency increases in a substantiallylinear fashion and the negative slip frequency which is utilized duringbraking of the induction motor will therefore vary linearly with motorspeed by the system that has been described.

With this arrangement that has been described and during the brakingmode of operation of the induction motor the output frequency of theconverter 24 will be a function of F,, F,,. This of course means thatthe frequency of the input voltage to the induction motor from theconverter 34 is less than a related frequency of the speed of rotationof the rotor 40 by the amount of negative slip frequency F,,. Thisfurther means that the induction motor 38 will now operate in aregenerative mode or as an induction generator since its input frequencyis less than a frequency related to the speed of rotation of the rotor40. This further means that induction motor 38 is now operating in abraking mode to slow down the rotor and the wheels 56 and 58 of thevehicle. It should be appreciated that the magnitude of the negativeslip frequency signal is determined by the instantaneous speed ofrotation of the rotor 40 of the induction motor as is depicted in FIG.6.

Referring now more particularly to FIG. 7, a system is illustrated whichis capable of inserting different valued resistors across the respectivephase windings of the induction motor 38 during different speed rangesof the rotor of the induction motor 38. The reason for the use of suchsystem is that it has been discovered that more efficient braking can beachieved with the system of this invention where different resistors areused over different predetermined speed ranges of the induction motor38. Thus, in the system of FIG. 1 resistors 42, 48 and 52 may be, forexample, 2.8 ohm resistors where the induction motor is to be brakedover a speed range of 4,000 to 12,000 rpm as shown in FIG. 6. Where itis desired to use different valued resistors for braking duringdifferent speed ranges of the motor the system of FIG. 7 can beutilized. FIG. 7 illustrates only a portion of FIG. 1 and the samereference numerals have been used as were used in FIG. 1 to identify thesame parts in each figure.

It is seen in FIG. 7 that the phase windings of the induction motor 38again are designated by the letters A, B and C. It is further seen thatthe phase windings are respectively connected in parallel with threeresistors which have been designated by reference numerals 310, 312 and314. These resistors can be paralleled by capacitors (not illustrated)as in FIG. 1. In the arrangement described the resistors 310 have thesame resistance value as do the resistors 312 and the resistors 314. Theresistance values of resistors 310, 312 and 314 are all difierenthowever and each is tailored to provide optimum braking during one ofthree speed ranges of the induction motor 38.

In FIG. 7 each resistor is shown connected in series with a relayoperated switch which is not identified by a reference numeral. Thecontacts located respectively in series with the resistors 310 arecontrolled by a relay coil designated as 310R in FIG. 7. In 'a similarfashion the contacts connected in series with resistors 312 arecontrolled by relay coil 312R and the contacts in series with resistors314 are controlled by relay coil 314R. All of the relay contacts forcontrolling the paralleling of the resistors with the phase winding areopen except when a respective relay coil is energized. For example, whenrelay coil 310R is energized the resistors 310 will each be connectedrespectively in parallel with a respective phase winding by a closure ofan associated relay contact.

Each of the relay coils 310R, 312R and 314R are connected to one side ofa source of direct current 316 through switch 104. The opposite side ofthe source of direct current is connected with movable electricalcontacts 310C, 312C and 314C. These contacts are mounted on a movableshaft 318 which is moved by a flyweight mechanism designated byreference numeral 320 and mechanically coupled to the rotor 40 of theinduction motor 38. The contacts 310C, 312C and 314C slidably engagefixed contacts 310D, 312D and 314D. With the arrangement shown in FIG. 7it will be appreciated that as the movable contacts 310C-314C areshifted by the centrifugal mechanism 320 one of the movable contactswill respectively engage a fixed contact over a predetermined speedrange of the motor 40. Thus, as shown in FIG. 7, relay coil 310R will beenergized for as long as the movable contact 310C is in en gagement withthe fixed contact 310D. As the shaft 318 is moved by the centrifugalmechanism the contact 310C will eventually leave fixed contact 310D andanother movable contact for example contact 312C will engage fixedcontact 312D. The arrangement is such that one relay coil is energizedover a given speed range and in the system shown in FIG. 7 each of thethree relay coils will be energized at different timesfor threedifierent speed ranges of the motor 38.

The relay coils 310R, 312R and 314R also respectively control relaycontacts 310B, 312B and 3148. Thus when relay coil 310R is energized itnot only connects resistors 310 in parallel with the respective phasewindings of the induction motor but also closes the relay contact 310B.The relay contact 310B is connected in series with a braking programmerdesignated as BPl. The braking programmer BPl determines the negativeslip frequency of the system and perfonns the same function as thebraking programmer 74 shown in FIG. 1 and will develop some outputvoltage which relates motor speed and negative slip frequency as shownin FIG. 6.

It is seen that relay contact 312B is connected in series with anotherbraking programmer BP2 and that relay contact 314B is connected withstill another braking programmer BP3. The braking programmers BPl, BP2and BP3 may all be arranged such that they develop different slopes forthe relationship between motor speed and negative slip frequency for agiven value of braking resistance which has been connected in parallelwith a given phase winding of the motor. It therefore will beappreciated that by use of the system shown in FIG. 7 a particularresistance value will be placed in parallel with a given phase windingof the induction motor to provide regenerative braking and theparticular resistance that is selected will be determined by theinstantaneous output speed of the induction motor 38.- In addition, itcan be seen that when a particular resistance is paralleled with a givenphase winding one of the switches 310B, 3128 or 3148 is closed toprovide the desired slip frequency characteristic for the givenresistance which is now being used for braking. The braking programmersBPl, BP2 and BPS provide linear output characteristics like that shownin FIG. 6 but have different motor speed-slip frequency functions for agiven speed range of the induction motor 38.

The operation of the motor control system of this invention will now bedescribed. Assuming first of all that the operator of the vehicle isoperating the vehicle in a motoring mode a voltage is applied to thephase windings A, B and C of the induction motor 38. During motoring thevehicle is controlled by an accelerator control designated by referencenumeral 320. This control is connected with voltage regulator 118 byline 322 and with converter 24 by line 324. During motoring theaccelerator control adjusts the output voltage of generator 12 and theduty cycle of the controlled rectifiers of converter 24 to control thevoltage applied to the motor. Since the brake pedal 106 is not depressedthe relay contactor 44D will be in its closed position shown in FIG. 1.This means that the slip frequency of the induction motor 38 will becontrolled by the summation of two direct voltages F,, F, which areadded in adder-subtractor 79 shown in FIG. 1. Since the converter 24will now be gated at a frequency which is a function of F,, F, the motor38 will operate at a constant slip frequency F, determined by themotoring programmer 76 shown in FIG. 1.

Assuming now that the operator of the vehicle desires to slow thevehicle down by braking the vehicle, the operator depresses the brakepedal 106. When the brake pedal 106 is depressed the relay coil 44 isenergized to shift contact 44D into engagement with contact 44F.In'addition the contacts 44A, 44B and 44C will all be closed to connectthe braking resistors 42, 48 and 52 in parallel with the respectivephase windings of the motor 38. The movement of the brake pedal 106performs one other function in that it causes the closure of relaycontacts 446 and 44H, shown in FIG. 4, and this as previously describedreduces the conduction angle of the controlled rectifiers in converter24 from 120 to 60.

When contact 44D moves from contact 44E to contact 44F the motor 38 ischanged from a positive slip frequency mode of operation to a negativeslip frequency mode of operation. This means thatthe converter 24 is nowgated at such a frequency that the frequency of the voltage applied tothe phase windings of the motor 38 is less than a frequency related tothe rotational speed of rotor 40 with a result, that the induction motor38 now operates as an induction generator in a regenerative mode. Duringthe time that the motor is operating with a negative slip frequency thisnegative slip frequency will be determined by the instantaneousrotational speed of the rotor 40 of the induction motor by arelationship illustrated in FIG. 6. Thus, as motor speed is increasedthe negative slip frequency is increased in a substantially linearfashion.

With the motor operating at a negative slip frequency voltages aregenerated in the phase windings A, B and C and currents are respectivelysupplied to the braking resistors 42, 48 and 52. The amount of currentflowing in the braking resistors will depend upon the instantaneousnegative slip frequency of the induction motor 38 and the output voltageof the converter 24. This output voltage is regulated by the voltageregulator 118 which in turn is controlled by variable resistor 116. Thismeans that during braking the voltage output of alternator l2 andtherefore the output voltage of converter 24 is related to the amountthe brake pedal 106 is depressed. This voltage is further held at aconstant value for a given setting of resistor 116 by the voltageregulator 118. Therefore as the brake pedal 106 is moved further in adepressed or braking direction the output voltage of the alternator isincreased but held at a constant value determined by the position of thebrake pedal 106.

One of the principal objects of this invention is to providesubstantially constant horsepower braking for a given position of thebrake pedal 106 and to provide an efficient arrangement where the .leastamount of power is supplied from the generator 12 to the brakingresistances 42, 48 and 52 during the braking mode of operation. In orderto effectuate these objects the effective resistance of the inductionmotor 38 is varied linearly by varying its negative slip frequency asshown in FIG. 6.

The effective resistance of an induction motor can be expressedapproximately as R R (fs/fm) where fs is the slip frequency, fm is theequivalent motor shaft frequency and R is the rotor resistance referredto the stator.

Since this is true its effective resistance can be maintainedsubstantially constant by varying the negative slip frequency of theinduction motor 38 as a function of motor speed, as shown in FIG. 6.This results in the effective resistance of the induction motor asviewed from its stator side substantially matching the resistance of thebraking resistors 42, 48 and 52. In other words, the effectiveresistance of, for example, the stator winding B of the induction motoris matched to the resistance of braking resistor 48 during the brakingmode of operation by continuously varying the negative slip frequency ofthe motor as shown in FIG. 6. This results in optimum and efficientbraking for the induction motor and results in a system where minimumpower is taken from the generator 12 during the braking operation andfurther results in a system where substantially constant horsepowerbraking is achieved over a given speed range of the induction motor 48.A factor in this constant horsepower braking, of course, is the factthat the output voltage of the generator 12 is made substantiallyconstant by the voltage regulator device 1 18 during the braking mode ofoperation. The braking power is proportional to the square of thegenerator voltage.

With the arrangement of FIG. 7, of course, different valued resistorsare connected in parallel with the respective phase windings of themotor for different speed ranges of the motor and the negative slipfrequency is again programmed according to a predetermined program tovary this negative slip frequency in a manner similar to that shown inFIG. 6 but with different braking programmers or braking programs foreach value of resistance utilized when the motor 38 is operating as aninduction generator.

The motoring slip frequency programmer 76 may take the form of anoperational amplifier or a function generator to provide the voltage F,,which as disclosed herein, is a function of motor speed F,,,. Themotoring slip frequency programmer may, however, have no relationship tomotor speed and could respond to other operating parameters of thesystem. Moreover, the motoring programmer may be arranged to provide thesame output voltage for all motor speeds in which case the motor willoperate at a constant slip frequency for all motor speeds.

It should be pointed out with reference to the modification of FIG. 7that relay coils 310R, 312R and 314R will not be energized until switch104 is closed and this switch is only closed when brake pedal 106 isdepressed. Moreover, the system of FIG. 7 can include another relay coil(not illustrated) which closes contacts 446 and 44H when switch 104 isclosed.

What is claimed is:

1. An induction motor control system for braking an induction motorcomprising, an induction motor having a winding and a rotor, a source ofalternating current, converter means connected between said source ofalternating current and said motor winding for applying an alternatingcurrent to said motor winding at a frequency determined by the switchingfrequency of said converter means, said converter means capable ofsupplying current from said source of alternating current to said motorwinding but operating in such a switching sequence as to prevent saidmotor winding from supplying current to said source of alternatingcurrent, at least one braking resistor, switching means for connectingsaid braking resistor in parallel with said winding of said motor,negative slip frequency control means connected with said convertermeans and responsive to the speed of rotation of the rotor of saidinduction motor for causing said induction motor to operate with apredetermined negative slip frequency when said negative slip frequencycontrol means is actuated, braking control means connected with saidswitching means and with said negative slip frequency control means,said braking control means when actuated operating said negative slipfrequency control means to cause said motor to operate at a negativeslip frequency while causing said switching means to connect saidbraking resistor across said output winding of said motor, and meansresponsive to the speed of rotation of said rotor for causing thenegative slip frequency of said motor to vary substantially linearlywith changes in rotor speed, said variation being such that saidnegative slip frequency decreases as motor speed decreases.

2. A motor control system for a polyphase induction motor comprising, aninduction motor having a polyphase winding and a rotor, a source ofalternating current, a converter including a plurality of switchingdevices connected between said source of alternating current and saidwinding of said motor, triggering means connected with said switchingdevices of said converter for triggering said switching devices at apredetermined frequency whereby the frequency of the output voltage ofsaid converter which is applied to said motor winding can be controlled,said triggering means controlling said converter such that current issupplied to said motor winding from said source of alternating currentwhile preventing current flow in a direction from said motor windingtoward said source of altemating current, a plurality of brakingresistors, means including switching means for selectively connecting arespective braking resistor in parallel with a respective phase windingof said motor, a negative slip frequency control means including meansfor sensing the speed of rotation of a rotor of said induction motor forcausing said converter to operate at such a frequency that saidinduction motor operates at a predetermined negative slip frequency asan induction generator, braking control means, means coupling saidbraking control means to said switching means and to said negative slipcontrol means whereby said braking resistors are connected in parallelwith said phase windings of said motor and said motor is operated in anegative slip frequency condition when said braking control device isactuated, means responsive to the speed of rotation of said motor forvarying the negative slip frequency of said motor substantially linearlywith changes in speed of said motor when said motor is operating in saidbraking mode, and means controlled by said braking control means forregulating the output voltage of said source of alternating current whensaid motor is operating in a braking mode.

3. A motor control system for a polyphase induction motor comprising, apolyphase induction motor having a polyphase winding and a rotor, asource of alternating current, converter means connected between saidsource of alternating current and said phase windings of said motor,said converter means comprised of a plurality of groups of controlledrectifiers, triggering means coupled to said groups of controlledrectifiers for causing said groups of controlled rectifiers to switch ina predetermined sequence to thereby apply a substantially square wavealtemating' voltage to said polyphase winding of said induction motor ata frequency determined by the switching frequency of said triggeringmeans, said triggering means switching said controlled rectifiers suchthat power cannot be returned to said source of alternating current fromsaid motor, a plurality of braking resistors one for each phase windingof said motor, means including switching means for selectivelyconnecting a braking resistor in parallel with a given phase winding ofsaid motor, brake control means, means coupling said brake control meanswith said switching means whereby said switching means is closed toconnect a respective braking resistor in parallel with a respectivephase winding of said motor when said brake control means is actuated, anegative slip frequency control means responsive to the speed ofrotation of said motor and coupled to said triggering means forcontrolling the output frequency of said converter means, said negativeslip frequency control means including means for triggering saidconverter at such a frequency relative to the rotat-.

ing speed of the rotor of said motor that said motor operates with anegative slip frequency as an induction 3 generator, means coupling saidbrake control means with said negative slip frequency control meanswhereby said negative slip frequency control means is put into operationwhen said brake control means is actuated, and means for causing thenegative slip frequency of said induction motor to vary substantiallylinearly with changes in rotor speed of said motor over a predeterminedspeed range of said motor including means for sensing the speed ofrotation of said rotor.

4. The motor control system according to claim 3 where each brakingresistor is paralleled by a capacitor.

5. A motor control system for an induction motor comprising, aninduction motor having a winding and a rotor, a source of alternatingcurrent, frequency control means connected between said source ofalternating current and said winding of said motor, said frequencycontrol means including a plurality of switching devices, triggeringmeans connected with said switching devices for determining thefrequency of occurrence and pulse width of the output voltage pulsesapplied to said motor winding from said frequency control device,braking resistor means, means including a switching means for connectingsaid braking resistor means in parallel with said winding of said motor,negative slip frequency control means coupled to said triggering meansand responsive to the speed of rotation of the rotor of said motor, saidnegative slip frequency control means operative to regulate theswitching frequency of said switching devices of said frequency controlmeans such that said induction motor operates as with a negative slipfrequency in a braking mode when said negative slip frequency controlmeans is actuated, angle control means for determining the pulse widthof the output voltage pulses applied to said motor winding from saidfrequency control device, a braking control device, and means couplingsaid braking control device with said switching means, with saidnegative slip frequency control means and with said angle control means,said braking control device when actuated causing said switching meansto connect said braking resistor in parallel with said output winding,actuating said negative slip frequency control means to operate saidmotor with a negative slip frequency and actuating said angle controlmeans to reduce the pulse width of the voltage pulses applied to saidmotor winding.

6. The motor control system according to claim where means are providedfor controlling the output voltage of said source of alternating currentand wherein said control means is regulated in response to actuation ofsaid braking control device. v

7. A motor control system for an induction motor having a polyphasewinding and a rotor comprising, a source of alternating current,converter means connected between said source of alternating current andsaid polyphase winding of said motor, said converter means including aplurality of switching devices, a triggering means connected with saidswitching devices for causing said switching devices to be triggeredconductive in a predetermined sequence and at a predetermined frequencyto thereby supply apolyphase alternating current to said polyphase motorwinding at a predetermined frequency, positive slip frequency controlmeans connectible with said triggering means and responsive to the speedof rotation of the rotor of said motor, said positive slip frequencycontrol means when actuated operating to cause said converter means tohave an output frequency such that said motor is operated at apredetermined constant positive slip frequency, negative slip frequencycontrol means connectible to said triggering means and responsive to thespeed of rotation of said rotor for causing the output frequency of saidconverter to be of such a value relative to the rotational speed of saidmotorthat said induction motor operates at a negative slip frequency ina braking mode, a braking resistor for each phase winding of saidpolyphase motor winding, braking switching means operable to selectivelyconnect a given braking resistor in parallel with a respective phasewinding of said motor, brake control means, means coupled to said brakecontrol means for connecting said negative slip frequency control meanswith said trigger means and disconnecting said positive slip frequencycontrol means from said trigger means when said brake control means isactuated, means coupling said brake control means and said brakingswitching means whereby said braking resistors are connected in parallelwith said phase windings of said motor when said brake control means isactuated, and means for causing the negative slip frequency of saidnegative slip frequency control means to increase substantially linearlywith increase in rotor speed, said last named means including means fordetecting the instantaneous speed of rotation of said motor rotor.

8. A motor control system for an induction motor having a polyphasewinding and a rotor comprising, a source of alternating current,frequency control means connected between said source of alternatingcurrent and said polyphase motor winding for controlling the frequencyof the voltage applied to said motor winding,

first control means coupled to said frequency control means for causingsaid motor to operate with a substantially constant positive slipfrequency, second control means coupled to said frequency control meansfor causing said motor to operate with a negative slip frequency in abraking mode, said second control means including means for causing thenegative slip frequency to vary as a function of motor speed, said firstand second control means including means sensing the speed of rotationof the rotor of said motor, braking resistor means for each phasewinding of said motor, switching means for selectively connecting arespective braking resistor in parallel with a respective phase windingof said motor, brake control means for causing said switching means toconnect said resistors respectively in parallel with said phase windingsof said motor and causing said second control means to control theoperation of said motor, means for varying the resistance value of saidbraking resistor means as a function of the speed of rotation of therotor of said motor, and means for changing the functional relationshipbetween motor speed and negative slip frequency of said second controlmeans in response to motor speed whereby the resistance value of saidbraking resistor means and negative slip frequency have a predeterminedrelationship.

9. A motor control system for a polyphase induction motor comprising, aninduction motor having a polyphase winding and a rotor, a source ofpolyphase alternating current, an alternating current to altematingcurrent converter means connected between said source of alternatingcurrent and said polyphase motor winding including a plurality ofpolyphase full-wave bridge rectifiers comprised of controlledrectifiers, triggering means coupled to said controlled rectifiers forcausing said controlled rectifiers to switch at a predeterminedfrequency to thereby apply a substantially square wave alternatingvoltage to said polyphase winding of said motor, positive slip frequencycontrol means connectible with said triggering means and responsive tothe speed of rotation of said rotor for causing said converter to switchat such a frequency that said motor is operated with a substantiallyconstant positive slip frequency, negative slip frequency control meansconnectible with said triggering means and responsive to the speed ofrotation of said rotor for causing said converter to switch at such afrequency related to rotor speed that said motor is operated with anegative slip frequency in a braking mode, braking resistors connectiblerespectively in parallel with a respective phase winding of said motorwinding, braking control means, means responsive to the actuation ofsaid braking control means for causing said resistors to be connected inparallel with said phase windings of said motor and for causing saidconverter to be controlled by said negative slip frequency controlmeans, and means for causing the negative slip frequency of said motorto increase substantially linearly with increasing rotor speed of saidmotor when said system is operating in said braking mode.

' 10. A vehicle propulsion system for a vehicle having at least onedriving wheel comprising, a prime mover, a polyphase alternating currentgenerator driven by said prime mover, a polyphase induction motor havinga polyphase winding and a rotor, means mechanically connecting the rotorof said induction motor with said driving wheel, an alternating currentto alternating current converter means connected between said outputwinding of said alternating current generator and said polyphase windingof said motor, said converter means including a plurality of controlledrectifiers connected in a plurality of polyphase fullwave bridgerectifier networks, triggering means coupled with said controlledrectifiers for causing said controlled rectifiers to switch at apredetermined frequency whereby a substantially square wave alternatingvoltage is applied to said motor winding from the output of saidconverter means, positive slip frequency control means coupled to saidtriggering means and responsive to rotor speed for causing saidconverter to operate at such a frequency that said motor operates with asubstantially regulated positive slip frequency, negative slip frequencycontrol means responsive to the speed of rotation of said rotor coupledto said triggering means for causing said converter to operate at such afrequency relative to the speed of said rotor that said induction motoris operated with a regulated negative slip frequency to provide abraking mode of operation, braking resistors connectible respectively inparallel with a respective phase winding of said 'motor, manuallyoperable braking control means, means coupling said braking controlmeans to said switching means and to said positive and negative slipfrequency control means such that said negative slip frequency controlmeans provides the frequency control for said motor when said brakingcontrol means is actuated, said braking control means causing saidswitching means to connect said resistors respectively in parallel withthe phase windings of said motor, and means responsive to the speed ofrotation of said rotor for increasing the negative slip frequency ofsaid motor substantially linearly with increasing speed of said rotor.

1. An induction motor control system for braking an induction motorcomprising, an induction motor having a winding and a rotor, a source ofalternating current, converter means connected between said source ofalternating current and said motor winding for applying an alternatingcurrent to said motor winding at a frequency determined by the switchingfrequency of said converter means, said converter means capable ofsupplying current from said source of alternating current to said motorwinding but operating in such a switching sequence as to prevent saidmotor winding from supplying current to said source of alternatingcurrent, at least one braking resistor, switching means for connectingsaid braking resistor in parallel with said winding of said motor,negative slip frequency control means connected with said convertermeans and responsive to the speed of rotation of the rotor of saidinduction motor for causing said induction motor to operate with apredetermined negative slip frequency when said negative slip frequencycontrol means is actuated, braking control means connected with saidswitching means and with said negative slip frequency control means,said braking control means when actuated operating said negative slipfrequency control means to cause said motor to operate at a negativeslip frequency while causing said switching means to connect saidbraking resistor across said output winding of said motor, and meansresponsive to the speed of rotation of said rotor for causing thenegative slip frequency of said motor to vary substantially linearlywith changes in rotor speed, said variation being such that saidnegative slip frequency decreases as motor speed decreases.
 2. A motorcontrol system for a polyphase induction motor comprising, an inductionmotor having a polyphase winding and a rotor, a source of alternatingcurrent, a converter including a plurality of switching devicesconnected between said source of alternating current and said winding ofsaid motor, triggering means connected with said switching devices ofsaid converter for triggering said switching devices at a predeterminedfrequency whereby the frequency of the output voltage of said converterwhich is applied to said motor winding can be controlled, saidtriggering means controlling said converter such that current issupplied to said motor winding from said source of alternating currentwhile preventing current flow in a direction from said motor windingtoward said source of alternating current, a plurality of brakingresistors, means including switching means for selectively connecting arespective braking resistor in parallel with a respective phase windingof said motor, a negative slip frequency control means including meansfor sensing the speed of rotation of a rotor of said induction motor forcausing said converter to operate at such a frequency that saidinduction motor operates at a predetermined negative slip frequency asan induction generator, braking control means, means coupling saidbraking control means to said switching means and to said negative slipcontrol means whereby said braking resistors are connected in parallelwith said phase windings of said motor and said motor is operated in anegative slip frequency condition when said braking control device isactuated, means responsive to the speed of rotation of said motor forvarying the negative slip frequency of said motor substantially linearlywith changes in speed of said motor when said motor is operating in saidbraking mode, and means controlled by said braking control means forregulating the output voltage of said source of alternating current whensaid motor is operating in a braking mode.
 3. A motor control system fora polyphase induction motor comprising, a polyphase induction motorhaving a polyphase winding and a rotor, a source of alternating current,converter means connected between said source of alternating current andsaid phase windings of said motor, said converter means comprised of aplurality of groups of controlled rectifiers, triggering means coupledto said groups of controlled rectifiers for causing said groups ofcontrolled rectifiers to switch in a predetermined sequence to therebyapply a substantially square wave alternating voltage to said polyphasewinding of said induction motor at a frequency determined by theswitching frequency of said triggering means, said triggering meansswitching said controlled rectifiers such that power cannot be returnedto said source of alternating current from said motor, a plurality ofbraking resistors one for each phase winding of said motor, meansincluding switching means for selectively connecting a braking resistorin parallel with a given phase winding of said motor, brake controlmeans, means coupling said brake control means with said switching meanswhereby said switching means is closed to connect a respective brakingresistor in parallel with a respective phase winding of said motor whensaid brake control means is actuated, a negative slip frequency controlmeans responsive to the speed of rotation of said motor and coupled tosaid triggering means for controlling the output frequency of saidconverter mEans, said negative slip frequency control means includingmeans for triggering said converter at such a frequency relative to therotating speed of the rotor of said motor that said motor operates witha negative slip frequency as an induction generator, means coupling saidbrake control means with said negative slip frequency control meanswhereby said negative slip frequency control means is put into operationwhen said brake control means is actuated, and means for causing thenegative slip frequency of said induction motor to vary substantiallylinearly with changes in rotor speed of said motor over a predeterminedspeed range of said motor including means for sensing the speed ofrotation of said rotor.
 4. The motor control system according to claim 3where each braking resistor is paralleled by a capacitor.
 5. A motorcontrol system for an induction motor comprising, an induction motorhaving a winding and a rotor, a source of alternating current, frequencycontrol means connected between said source of alternating current andsaid winding of said motor, said frequency control means including aplurality of switching devices, triggering means connected with saidswitching devices for determining the frequency of occurrence and pulsewidth of the output voltage pulses applied to said motor winding fromsaid frequency control device, braking resistor means, means including aswitching means for connecting said braking resistor means in parallelwith said winding of said motor, negative slip frequency control meanscoupled to said triggering means and responsive to the speed of rotationof the rotor of said motor, said negative slip frequency control meansoperative to regulate the switching frequency of said switching devicesof said frequency control means such that said induction motor operatesas with a negative slip frequency in a braking mode when said negativeslip frequency control means is actuated, angle control means fordetermining the pulse width of the output voltage pulses applied to saidmotor winding from said frequency control device, a braking controldevice, and means coupling said braking control device with saidswitching means, with said negative slip frequency control means andwith said angle control means, said braking control device when actuatedcausing said switching means to connect said braking resistor inparallel with said output winding, actuating said negative slipfrequency control means to operate said motor with a negative slipfrequency and actuating said angle control means to reduce the pulsewidth of the voltage pulses applied to said motor winding.
 6. The motorcontrol system according to claim 5 where means are provided forcontrolling the output voltage of said source of alternating current andwherein said control means is regulated in response to actuation of saidbraking control device.
 7. A motor control system for an induction motorhaving a polyphase winding and a rotor comprising, a source ofalternating current, converter means connected between said source ofalternating current and said polyphase winding of said motor, saidconverter means including a plurality of switching devices, a triggeringmeans connected with said switching devices for causing said switchingdevices to be triggered conductive in a predetermined sequence and at apredetermined frequency to thereby supply a polyphase alternatingcurrent to said polyphase motor winding at a predetermined frequency,positive slip frequency control means connectible with said triggeringmeans and responsive to the speed of rotation of the rotor of saidmotor, said positive slip frequency control means when actuatedoperating to cause said converter means to have an output frequency suchthat said motor is operated at a predetermined constant positive slipfrequency, negative slip frequency control means connectible to saidtriggering means and responsive to the speed of rotation of said rotorfor causing the output frequency of said converter to Be of such a valuerelative to the rotational speed of said motor that said induction motoroperates at a negative slip frequency in a braking mode, a brakingresistor for each phase winding of said polyphase motor winding, brakingswitching means operable to selectively connect a given braking resistorin parallel with a respective phase winding of said motor, brake controlmeans, means coupled to said brake control means for connecting saidnegative slip frequency control means with said trigger means anddisconnecting said positive slip frequency control means from saidtrigger means when said brake control means is actuated, means couplingsaid brake control means and said braking switching means whereby saidbraking resistors are connected in parallel with said phase windings ofsaid motor when said brake control means is actuated, and means forcausing the negative slip frequency of said negative slip frequencycontrol means to increase substantially linearly with increase in rotorspeed, said last named means including means for detecting theinstantaneous speed of rotation of said motor rotor.
 8. A motor controlsystem for an induction motor having a polyphase winding and a rotorcomprising, a source of alternating current, frequency control meansconnected between said source of alternating current and said polyphasemotor winding for controlling the frequency of the voltage applied tosaid motor winding, first control means coupled to said frequencycontrol means for causing said motor to operate with a substantiallyconstant positive slip frequency, second control means coupled to saidfrequency control means for causing said motor to operate with anegative slip frequency in a braking mode, said second control meansincluding means for causing the negative slip frequency to vary as afunction of motor speed, said first and second control means includingmeans sensing the speed of rotation of the rotor of said motor, brakingresistor means for each phase winding of said motor, switching means forselectively connecting a respective braking resistor in parallel with arespective phase winding of said motor, brake control means for causingsaid switching means to connect said resistors respectively in parallelwith said phase windings of said motor and causing said second controlmeans to control the operation of said motor, means for varying theresistance value of said braking resistor means as a function of thespeed of rotation of the rotor of said motor, and means for changing thefunctional relationship between motor speed and negative slip frequencyof said second control means in response to motor speed whereby theresistance value of said braking resistor means and negative slipfrequency have a predetermined relationship.
 9. A motor control systemfor a polyphase induction motor comprising, an induction motor having apolyphase winding and a rotor, a source of polyphase alternatingcurrent, an alternating current to alternating current converter meansconnected between said source of alternating current and said polyphasemotor winding including a plurality of polyphase full-wave bridgerectifiers comprised of controlled rectifiers, triggering means coupledto said controlled rectifiers for causing said controlled rectifiers toswitch at a predetermined frequency to thereby apply a substantiallysquare wave alternating voltage to said polyphase winding of said motor,positive slip frequency control means connectible with said triggeringmeans and responsive to the speed of rotation of said rotor for causingsaid converter to switch at such a frequency that said motor is operatedwith a substantially constant positive slip frequency, negative slipfrequency control means connectible with said triggering means andresponsive to the speed of rotation of said rotor for causing saidconverter to switch at such a frequency related to rotor speed that saidmotor is operated with a negative slip frequency in a braking mode,braking resistors connectible respEctively in parallel with a respectivephase winding of said motor winding, braking control means, meansresponsive to the actuation of said braking control means for causingsaid resistors to be connected in parallel with said phase windings ofsaid motor and for causing said converter to be controlled by saidnegative slip frequency control means, and means for causing thenegative slip frequency of said motor to increase substantially linearlywith increasing rotor speed of said motor when said system is operatingin said braking mode.
 10. A vehicle propulsion system for a vehiclehaving at least one driving wheel comprising, a prime mover, a polyphasealternating current generator driven by said prime mover, a polyphaseinduction motor having a polyphase winding and a rotor, meansmechanically connecting the rotor of said induction motor with saiddriving wheel, an alternating current to alternating current convertermeans connected between said output winding of said alternating currentgenerator and said polyphase winding of said motor, said converter meansincluding a plurality of controlled rectifiers connected in a pluralityof polyphase fullwave bridge rectifier networks, triggering meanscoupled with said controlled rectifiers for causing said controlledrectifiers to switch at a predetermined frequency whereby asubstantially square wave alternating voltage is applied to said motorwinding from the output of said converter means, positive slip frequencycontrol means coupled to said triggering means and responsive to rotorspeed for causing said converter to operate at such a frequency thatsaid motor operates with a substantially regulated positive slipfrequency, negative slip frequency control means responsive to the speedof rotation of said rotor coupled to said triggering means for causingsaid converter to operate at such a frequency relative to the speed ofsaid rotor that said induction motor is operated with a regulatednegative slip frequency to provide a braking mode of operation, brakingresistors connectible respectively in parallel with a respective phasewinding of said motor, manually operable braking control means, meanscoupling said braking control means to said switching means and to saidpositive and negative slip frequency control means such that saidnegative slip frequency control means provides the frequency control forsaid motor when said braking control means is actuated, said brakingcontrol means causing said switching means to connect said resistorsrespectively in parallel with the phase windings of said motor, andmeans responsive to the speed of rotation of said rotor for increasingthe negative slip frequency of said motor substantially linearly withincreasing speed of said rotor.